Cullen's deep voice has earned him an active living in animation, commercial announcing, and trailer voice-overs. He may be best known as the voice of Optimus Prime, and others, in the animated series Transformers, reprising the role for the 2007 live-action film. Since 1988, he has been the primary voice of Eeyore in Disney'sWinnie the Pooh franchise (with Gregg Berger occasionally filling in). Other animation credits include DuckTales (Admiral Grimitz and Bankjob Beagle), Dungeons & Dragons (Venger), Rainbow Brite (Murky), and Challenge of the Go-Bots (Spoiler and others). Live-action roles include the voice of King Kong in the 1976 film version, several of the title characters in Gremlins, and K.I.T.T.'s rival K.A.R.R. in an episode of Knight Rider.